I'll have to copy Monochromatic here: Not to rain on your parade, Eldritch, but the Nightmares don't have invisibility spells :/ They're shadows, so to speak, and can hide in them simply because they look like a shadow themselves. That coupled with their ability to travel through solid objects gives them an illusion of being able to turn invisible. 

Also... A blood sacrifice for those thingies... Am I right to assume that said blood is coursing through them, then?